Composition for rectifying halide salt baths



Patented June 6, 1950 COMPOSITION roe eeo 'rrrrmo HAliIDE -'SiiIi l BATHS f-Isaa-c Laird Nowell, Wethersfleld, Conn., assignor to Heatbath Corporation, Indian Orchard, Mass, a corporafion at Massachusetts Nit-Drawing; zi ucatiano tcbra 1947, I Serial No. 777,848

This invention relates to a method of maintaining' the efllciencynf salt'baths' whichare composed substantially or halides and are commonly used in the treatment of certain metals. By 'halidesiin the present disclosure are meant the elements fluorine, chlorine, bromine, and iodine of group 7 of the periodic table.

- suchsalt baths as are composed of mixtures or :alkali metal and alkaline earth halides are commonly fused and used as media for the: heat treatment ,of steels and other 'rnetals. Itis well known in the case of :steel that after .a certain period of use a hathfhecomes decarhurizmg. to them etal. Gne object :of'this invention is to provide a simple, inexpensive-and effective method tor maintaining the efiiciency of the salt baths to eliminate decarburizaticn and do this without the disadvantages attending prior :art proc'dures so fares-they are known: to me.-

Said salt baths are also used in the treatment or copper alloys and. nickel alloys and "when used tor acertain period these metals become at talineto a degree that will c'ausepitting, etching and-staining of the metal being treated.

In. maintaining the efliciency of halide baths it is common in the language of the trade to speak of keeping a hath'neutral-to metal. 'In other words a neutral bath will neither *carburize nor-decarburiz'e steel, nor will it cause pitting and etching of other metals which :are'ecommonly treated in said baths. The term neutral as used in this disclosure is intended to have this :meaning. Further, the process of maintaining this type 'o'if loath neutral is called a process of reetification.

When such'haths are no longer neutral, it is customary to discard the baths or to treat the baths with materials such as silica or silicon car-- 11 Claims. (CL 252*71 borate.

bide which will cause the formation of insoluble materialswhich are then removed" as a sludge from the molten salt, but with "attendant loss of salt along with the sludge. This method also limits production as the de-s'ludg'ing usually "requires lowering the temperatm-e below the workmg temperature. The baths also increase in viscosity due to the materials added. H Another method of treatment heretofore used consists in immersing a carbon rod in the molten bath. This method does not give complete rectifi'catio'n under allconditions and presents some hazards since small pieces of carbon breaking from this rod may come in contact with the metal being treated and cause hard spots, or even "fused metal in the high temperature My invention makes use of materials which completely neutralize salt baths and yet are not open to the objections noted above.

I'have found that a molten salt bathv such as is composed substantially of alkali halides, alkaline earth halides, or a, mixture of alkali and alkaline earth halides which. hasbecomeza-lkalinc and thus'decarbmvizing to steel can be and :made neutral and nondecarburizin-g to -steel by treating the molten salt baths with a compoimdanongaseous-atstandard temperatures and pressures; substantially inert with respect to the metals :bemg treated, but reactive with their oxides, containing fluorine as a component ele ment and at least one -other element capable of forming an oxideneutral or acidic in reaction. By-standardtemperature andepressure is meant 0 Centigrade temperature and 760 millimeters pressure.

Among nongaseous"compoundsfcontaining fluorine found to be suitable as rectifiers of molten salt baths are those containing boron as one of the elements such 'as ammonium fiuoborate; viso- 'amm fiuoborate; potassium fiuoborate: lithium fiuoborate; calciumfluobora'te; magnesium fluoher-ate; strontium fiuoborate and barium *Gthe'r nongaseous fluorine compoimiis suitable as rectifiersimay have silicon vas one of the elements such as ammonium fluosilicat'e: dium" "fluo'si l'lcate and potassium fluosilicate". Suitable compounds containing fluorine -may also contain carbon such as polytetrailuoroethane. These materials are not intended to-be 'apcomplete list of the compounds inchided within the scope of this invention, but are given merely to show the wide variety of compounds that act satisfactorily as rectihers.

It is believed that the mechanism of 'r'ectifica ti-on takes place in 'thevrol'lowing manner:

(1) The compound containing fluorine reacts selectively with the oxides present to reduce them to the elemental state.

(2) The newly formed metals react with the fluorine .to .form new stable neutral compounds which willv .be either volatile at this bath temperature or soluble in the .molten'bath. v

(3) The new oxides'formed are either neutral or acidic in. reaction, and when acidic, they fur-'- ther react with any alkali present to form a new neutral compound.

.Herewith are several examples within the scope oi thisinvention.

Example I salt bath used for the hardening oi a stee'i, the composition of which is carbon- 0.45%, mangan'ese "6775a,, phosphorus 6.035%, and sulfur 05040%, is composed of 45% potassiumchloride and sodium chloride and is rectified by the use of potassium fluoborate. salt bath is fused by heating to the "hardening temperature of the steel, 15-25 R, "and when chemical analysis shows the bath to have an excessive alkalinity, which is above approximately 015% when cal culated as sodium-oxide, the bath is rectified adding potassium fluoborate. This is added small quantities with stirring. Suflicient potas slum fluoborate is added to reduce the alkalinity Example II A salt bath composed of 77% barium chloride and 23% sodium chloride is used to harden a steel composed of carbon 0.40%, manganese 0.75%, nickel 1.25% and chromium 0.60%, by heating it at a temperature of 1500 F. in the bath and quenching the steel in oil. The salt bathis kept in a neutral condition by rectifying with'polytetrafiuoroethane when chemical analysis-shows that the alkalinity of the bath is excessive. The polytetrafluoroethane is added to the molten bath by mechanically subme'rging small portions at a time in the bath. Suflicient polytetrafiuoroethane is added to neutralize the bath and the process is repeated as often as nec- 'ssary to keep the bath neutral. When the salt bath is kept neutralized as described, an article of steel having the above composition and having a surface free from pits and decarburization will when heated for an hour at 1500 F. show no material change in the surface quality.

Example III 1A:salt bath composed of anhydrous barium chloride is used to harden a tool steel containing 0173'% carbon; 0.30% manganese, 4.51% chromium, 18.21% tungsten, 0.75% molybdenum, 0.26% silicon, 1.25% vanadium by heating at a temperature of 2350 F. The barium chloride is kept in a neutral condition with respect to the toolsteelby rectifying the molten bath with sodium fluosilicate when the alkalinity becomes excessive, about 3% calculated as barium oxide. The sodium fluosilicate is added in small portions until .tests indicate that the alkalinity is neutrali'zed. After heating the tool steel for an hour at 2350 F. in the barium chloride kept neutral as described, the surface will be found to be substantially the same as the original from the standpoint "of decarburization and surface defects.

"' Example IV A salt bath composition for the heat treatment of steel is kept constantly neutral and nondecarburizing to the steel by the inclusion of sodium fluoborate in the following mixture, viz. barium chloride 55 parts; potassium chloride 25 parts; sodium chloride 20 parts and sodium fluoborate 6 partall parts taken by weight.

Example V A' salt'bath composition for the heat treatment of copper is kept constantly neutral to the copper and avoids pitting and etching by the inclusion of sodium fluoborate in the following mixture, viz. barium chloride 55 parts, potassium chloride 25 parts, sodium chloride 20 parts and sodium fluoborate part-all parts taken by weight.

What I claim is: I

1. A new composition of matter for the heat treatment of steels, copper alloys and nickel a1- loys, consisting of barium chloride 55 parts; potassium chloride 25 parts; sodium chloride 20 parts and sodium fluoborate part, all taken by weight.

- 2. A composition of matter for the heat treatment of metal consisting of a mixture of halide salts together with a compound added in a proportion of at least part compound to 100 parts halide salts, said compound being nongaseous at standard temperature and pressure and consisting of three elements, one of which is fluorine, another which is taken from the alkali and alkaline earth metal group and a third element chosen from a group consisting of boron,

carbon, and silicon.

3. A composition of matter for the heat treatment of steel in a bath consisting of a mixture of halide salts with at least part an alkali metal fluoborate salt, non-gaseous at standard temperature and pressure, to parts halide salts.

4. A composition of matter for the heat treatment of steel in a bath consisting of a mixture of halide salts with at least T 0- part an alkali-earth metal fluoborate salt, non-gaseous at standard temperature and pressure, to 100 parts halide salts.

5. A composition of matter for the heat treatment of steel in a bath consisting 'of a mixture of alkali and alkaline earth halides with at least 9 part an alkali metal fluoborate salt, nongaseous at standard temperature and pressure; to 100 parts alkali and alkaline earth halides.

6. A composition of matter for the heat treatment of steel in a bath consisting of a mixture of alkali and alkaline earth halides with at least part an alkali earth metal fluoborate salt, non-gaseous at standard temperature and pressure, to 100 parts alkali and alkaline earth halides.

7. A composition of matter for the heat treatment of steel in a bath consisting of a mixture of alkali halides together with at least part an alkali metal fluoborate salt, non-gaseous at standard temperature and pressure, to 100 parts alkali halides.

8. A composition of matter for the heat treatment of steel 'in a bath consisting of a mixture of alkali halides with at least part an alkaliearth metal fluoborate salt, non-gaseous at standard temperature and pressure, to 100 parts alkali halides.

9'. A composition of matter for the heattreat ment of steel in a bath consisting of a mixture of alkaline earth halides with at least part of an alkalimetal fluoborate salt, non-gaseous at standard temperature and pressure, to 100 parts alkaline earth halides.

' 10. A composition of matter for the heat treat-' ment of steel in a bath consistingof a mixture of alkaline earth halides with at least part of an alkali-earth metal fluoborate salt, nongaseous at standard temperature and pressur to 100 parts alkaline earth halides. I

11. A composition of matter for the heat treatment of steel in a bath consisting of a mixture of halide salts with at least part an alkali metal fluosilicate, non-gaseous at standard temperature and pressure, to 100 parts halide salts.

ISAAC LAIRD NEWELL.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 2,394,777 Holden et a1. Feb. 12, 1946 2,400,511 ,Holden May 21, 1946 

1. A NEW COMPOSTION OF MATTER FOR THE HEAT TREATMENT OF STEELS, COPPER ALLOYS AND NICKEL ALLOYS, CONSISTING OF BARIUM CHLORIDE 55 PARTS; POTASSIUM CHLORIDE 25 PARTS; SODIUM CHLORIDE 20 PARTS AND SODIUM FLUOBORATE 1/10 PART ALL TAKEN BY WEIGHT. 